The present invention relates in general to communication systems and components therefor, and is particularly directed to a new and improved mechanism for preventing clipping of pulse metering (teletax) signals in a telephone line card channel that results from the differential impedance between a subscriber line interface circuit (SLIC) and the line at the frequency band of teletax signals. The invention mitigates against or effectively eliminates clipping by tracking the pulse metering signals applied to the SLIC stage by means of a teletax signal cancellation circuit. The teletax signal cancellation circuit contains a transconductance amplifier circuit of the type described in the above-referenced ""408 application, and generates a signal current that is fed back to the transmission channel path of the SLIC in a manner that effectively cancels the reflected teletax signal flowing through a programmed impedance element installed in the transmission channel.
As described in the above-referenced ""408 and ""* * * applications, the transmission channels of subscriber line interface circuits, or SLICs, employed by telecommunication service providers to interface a communication wireline pair with subscriber (voicexe2x80x94data) communication equipment, must conform with a very demanding set of performance requirements. These requirements typically encompass accuracy, linearity, insensitivity to common mode signals, low power consumption, low noise, filtering, and ease of impedance matching programmability, in order to facilitate interfacing the SLIC with a variety of telecommunication circuits including those providing digital codec functionality.
In a typical application, the length of the wireline pair to which a SLIC is connected can be expected to vary from installation to installation, and may have a very significant length (e.g., on the order of multiple miles), transporting both substantial DC voltages, as well as AC signals (e.g., voice and/or ringing). As a consequence, it has been difficult to realize a SLIC implementation that has xe2x80x98universalxe2x80x99 use in both legacy and state of the art installations.
Advantageously, the SLIC transmission channel described in the above-referenced ""505 application successfully satisfies these requirements by a new and improved combination of a front end, current-sensing, voltage-feed transimpedance stage that is coupled in cascade with a transconductance amplifier-configured filter/gain output stage. The current-sensing, voltage-feed transimpedance stage transforms differentially sensed tip and ring input currents of a telecommunication wireline pair into a precise, single ended output voltage. This single ended output voltage is transformed by the transconductance amplifier-based filter/gain output stage into a very precise, single ended output current, and converted into a single ended output voltage for application to downstream voltage-fed circuitry. In addition, the transmission channel is configured to have the output impedance it presents to the line programmable by means of a single programming pin.
Irrespective of the type of SLIC that may be installed in various telephone line cards of a telecommunication service provider""s equipment facility, it is a relatively common practice worldwide, and the subject of ongoing demand to regulatory agencies by service providers in the United States, to tariff (or tax) any customer access (including xe2x80x98localxe2x80x99 calls) to a service provider""s equipment on a time of usage basis. For this purpose, it has been conventional practice to employ a frequency-diversity technique known as pulse metering, or xe2x80x98teletaxxe2x80x99, in which out-of-band, pulse-burst signals (typically in a range of 12 to 16 KHz) are superimposed on the in-band voice channel signals (on the order of up to 3 to 4 KHz) that are interfaced by the line circuit.
Although the basic signal transport and separation functionality of teletax signalling has worked reasonably well in the past, it can introduce significant impairments to the throughput and operation of more recently developed line cards that are configured to conform with an increasingly stringent set of parameters, particularly relatively constrained operating positive power supply voltages (on the order of three volts or less and declining).
The fundamental problem is the difference in output impedance the SLIC presents to the line for the two ranges of signals (in-band voice at 3-4 KHz and pulse metering at 12-14 KHz). In order to comply with the basic requirement of optimizing voice frequency transmissions, the SLIC is customarily configured to synthesize the output impedance Zo it presents to (the tip and ring terminals of) the telephone line, such that Zo equals the xe2x80x98characteristic impedancexe2x80x99 ZL of the line. While this impedance may vary among different countries, a value of Zo=600 ohms is relatively common.
With successful synthesis of the SLIC""s output impedance to match that of the line (i.e., Zo=ZL), the open loop differential voice signal Vtr appearing across the line""s tip (T) and ring (R) leads will be attenuated by a factor of two. Namely,
Vtr=Vsignalo1*ZL/(ZL+Zo)=Vsignalo1/2.xe2x80x83xe2x80x83(1)
However, at the higher teletax frequencies, the impedance in the line decreases considerably; a typical value of line impedance ZL at 12-14 KHz may be on the order of only 200 ohms. Substituting this value of line impedance in equation (1) results in a doubling of the attenuation factor of Vtr from one-half to one-fourth.
Because of this increased attenuation, the amplitude of the pulse-metering signals are normally increased (e.g., doubled) to ensure proper levels of reception at the receiving end of the circuit. These larger amplitude signals are reflected back to the impedance synthesis loop of the SLIC, whose transmission port is connected to a coding channel in an associated codec, which is companion to the SLIC in the line card. As codecs are usually very low voltage devices, they are incapable of handling large amplitude signals. As a consequence, the reflected teletax signals can lead to distortion in the codec due to clipping.
In accordance with the present invention, such clipping and distortion associated with the differential impedance between the SLIC and the line at the frequency band of teletax signals is effectively eliminated by tracking the pulse metering signals applied to the teletax port of a SLIC stage, using a teletax signal cancellation circuit that incorporates a transconductance amplifier circuit of the type described in the above-referenced ""408 application. The teletax signal, which is sensed by this cancellation circuit through a resistorxe2x80x94capacitor delay circuit, is used to generate a signal current that is fed back to the transmission channel path of the SLIC stage in a manner that effectively cancels the reflected teletax signal flowing through a programmed impedance element.
As will be described, the configuration of the transconductance amplifier circuit and current mirror ratios of current mirror circuitry it employs are such as to provide a prescribed attenuation Attx that is effective to provide for cancellation of teletax signal current. In particular, the parameters of the cancellation circuit are defined such that for teletax frequency band signals, the synthesized output impedance Zo of the SLIC has a value of zero and thereby eliminates the presence of pulse-metering signals at the input of a codec, so as to preclude clipping. In order to quantify the resistors and the capacitor of the delay circuit at values that provide complete cancellation of the reflected teletax signal, the peak amplitude of the differential line voltage is made equal to the voice/data overload voltage.